The accessibility of single-stranded sequences in 16S RNA in free state and in ribonucleoprotein particles (RNP) to complementary binding with isoplith fractions of oligonucleotides was studied. RNP had different protein composition and corresponded to intermediate stages of E. coli 30S subunit assembly in vitro. Gel-filtration was used to detect the most strong binding. It was found that S4 essentially inhibited the hexamer binding to RNA. 'Core' proteins bound to 16S RNA strongly increased the shielding of single-stranded regions while 'split' proteins insignificantly changed the hexamer binding. Nevertheless evidence is presented that 'split' proteins might also interact directly with 16S RNA in the 30S subunit.